Trigonometrical questions can be tricky if not understood properly.
The key concept to solving trigonometrical functions is the understanding of quadrant.
The sine, cosine and tangent operation and their numerical values are much influenced by the position their angles made.
This video shows you the Quadrant method used to handle these solving.
It deals with this matter nicely. So stay on with this simple presentation, and all will be fine with trigonometry.